Project Summary. The phenotype of angiotensin converting enzyme (ACE) knockout mice is a complex combination of cardiovascular, reproductive, hematologic and renal defects. This diversity of abnormalities supports the idea that ACE and angiotensin II influence many physiologic processes beyond simple blood pressure control. ACE is a zinc metalloproteinase composed of two regions termed N- and C-terminal domains. These domains are highly homologous but have somewhat different properties. In particular, while both domains can cleave angiotensin I, the tetrapeptide AcSDKP is exclusively cleaved by the N- domain. To investigate the in vivo roles of each domain, I created two new strains of mice. These mice were generated by homologous recombination in order to mutate the specific amino acids responsible for zinc binding (and thus catalysis) in each domain. In mice termed N-KO, the N-terminal domain of ACE is inactivated. In C-KO mice, it is the C-terminal domain that is no longer catalytic. The cardiovascular and renal phenotypes of N-KO and C-KO mice are indistinguishable from this of wild-type. These mouse models are ideal to study specific functions of ACE independent of blood pressure changes, a typical bias in studying the RAS with pharmacologic manipulations. My first aim is to complete the characterization of the C-KO mice. This investigation will be performed during the mentored phase of this grant. The next aims will be fully developed during the independent phase. The second aim is to investigate the in vivo function of the two catalytic sites of ACE in the progression of pulmonary fibrosis using bleomycin-induced lung injury as a model. My hypothesis is that ACE controls the concentration of both pro-fibrotic (angiotensin II) and anti-fibrotic (AcSDKP) molecules. My preliminary data strongly suggest that the N-KO mice are protected against bleomycin-induced lung injury. I find this in studies using both low and high doses of bleomycin. The third aim is to identify the mechanism responsible of this resistance. My hypothesis is that AcSDKP, elevated in N-KO mice, inhibits the progression of lung fibrosis. Relevance. This proposal will yield fundamental knowledge about the importance of ACE and its multiple substrates in tissue injury. Finally, my studies are both interesting and relevant to a variety of human diseases, including diseases of the lung, heart and kidney.